A person (almost always a female) who tries to be as good and "clean" as humanly possible. She is more often than not a staunch conservative and takes pride in her virginity and her practice of abstinence. She is definitely a God-fearing girl who always goes to church every Sunday, and indeed, based on the way she dresses, she looks like she's going to church every day. She cannot abide it when people cuss in front of her–the most extreme goody two shoeses faint when hearing foul language–and of course she would never consider smoking, drinking, doing drugs, or having any physical contact with a boy beyond holding hands or perhaps a kiss on the cheek. Can be nice but eventually begins to lecture you about your "sinful" lifestyle and just becomes a pest.

"Oh no, she wouldn't even think of going to that house party. She's such a little goody two shoes."

(noun)
Belittling and insulting term for a person who won't do anything that can be regarded as "wrong" or "immoral". A male or female who perceives him/herself (or is believed to perceive him/herself) as morally superior - often also God-fearing and full-bloodedly American.
(Disclaimer: Not very urban or very modern slang these days, though.)

A person who always does the "right thing", never does anything that they're "not supposed to do" and acts "perfect" in every single way possible. Usually a very peppy/happy person who is a "go getter". Also usually a girl. These people can be very annoying because of these traits.

Omg, that girl is such a goody two-shoes. I hate the way she acts like the teachers pet.